Na+and K+ion mobilities in thermally grown SiO2films have been determined from transient ion curret measurements in MOS capacitors. The mobilities were determined from the observed transit times of Na+and K+ions measured in the temperature ranges 40–180 °C and 230–300 °C, respectively. For Na+ions in both a dry‐grown oxide and one grown in a 10% HCl/oxygen mixture, &mgr;=1.0 exp(−0.66eV/kT) cm2/V sec. For K+ions in a dry‐grown oxide, &mgr;=0.03 exp(−1.09eV/kT) cm2/V sec.